Services for delivering digitized content such as movies and music over the Internet or the like have been in use in recent years. Such content is encrypted by Digital Rights Management (DRM) technology before delivery. DRM technology protects the content against illegally copying and distributed. However, there are many instances in which content viewers illegally copy video content by capturing the video content displayed on a computer display or a television monitor by using an image capturing apparatus such as a video camera and illegally distribute the copied video content. Since the video content displayed on the display is not encrypted, it is difficult to prevent the distribution of the video content copied by capturing the displayed video.
In view of the above, a technology has been developed that embeds information such as a viewer identification number as a digital watermark before delivering the content. The digital watermark remains intact in the copied video content even when the video content with the embedded digital watermark is illegally copied, for example, by capturing the displayed video content using a video camera. Therefore, even after the video content illegally copied by such analog capture is distributed, the identification number of the viewer that has made illegal use of the video content can be identified by detecting the digital watermark from the copied video content. Accordingly, even when the video content illegally copied by analog capture is uploaded to a website or the like, an administrator of the video content can identify the illegal copy source from the information embedded in the uploaded video content.
Furthermore, the digital watermarking technique may be applied to digital signage. For example, by detecting digital watermark information from video obtained by capturing digital video for advertisement displayed on a street screen or a television into which the digital watermark information is embedded, a user may obtain additional information such as detailed information of a product introduced by the digital video for advertisement.
In video data re-digitized by capturing video content displayed on a display using a capturing apparatus, a region of an image included in the original video content may be expanded or shrunk, or a position of the image region may be shifted at the time of capturing. Therefore, in such re-digitalized video data, a size and a position of a region in which digital watermark information is embedded may be different from the original size and position. A technique to detect edges of a rectangular image within an image is proposed to accurately detect digital watermark information (for example, refer to Japanese Laid-Open Patent Publication No. 2005-277732). In addition, a technique for changing a luminance value of a pixel at a predefined position of each picture of video data in a predetermined pattern along the time axis is proposed (for example, refer to Japanese Laid-Open Patent Publication No. 2005-277732 or Suzuki et al. (2009) Method for specifying spatio-temporal coordinate of digital watermark against re-captured image. Eighth Forum on Information Technology 2009 I-033, 307-308).
However, edges of the original rectangular image may not be captured within an image included in re-digitized video data or luminance variations among adjacent pixels at the edges may not be apparent. In such a case, an apparatus for detecting digital watermark information is unable to detect the edges of the rectangular image and, as a result, may not accurately identify the region into which digital watermark information was embedded. Furthermore, in order to detect a position of a pixel with such a luminance value variation by checking a luminance value variation of each pixel, images including at least one cycle of a pattern of a luminance value variation may be required. However, when an illegal user captures video content using a video camera, a position of the video camera may change due to a camera shake or the like, which also leads to a change in the position of the pixel to which a luminance value variation among images is added. As a result, an apparatus for detecting digital watermark information may not accurately detect the position of the pixel to which a luminance value variation is added.